CWG2886 [class.temporary] noexcept operator with trivial but potentially-throwing special member functions #531

Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [class.temporary]

Link to reflector thread (if any):

Issue description:

[class.temporary] p4 seemingly allows, but does not guarantee, that a trivial special member function is not called in the mentioned cases. As P1286R2 allowed trivial, eligible/non-deleted, but potentially-throwing (noexcept(false)) special member functions, the difference on whether such a special member function is called can be observed by a noexcept-expression.

There is implementation divergence on this (Godbolt link). It's unclear whether the implementation divergence is intended.

struct TrivialButPotentiallyThrowingDestructor {
    TrivialButPotentiallyThrowingDestructor() = default;
    ~TrivialButPotentiallyThrowingDestructor() noexcept(false) = default;
};

static_assert(noexcept(TrivialButPotentiallyThrowingDestructor{}), "???"); // EDG thinks the dtor is called, while others don't.

Also, it is unclear in [class.temporary] p3 whether the constructor or destructor call in the creation of a temporary object holding the function parameter is considered observable via a noexcept-expression. It seems intended that the implementation should only perform bitwise copy and the exception specification shouldn't be considered.

Suggested resolution: